sigprocmask() is used to fetch and/or change the signal mask of the
calling thread. The signal mask is the set of signals whose delivery
is currently blocked for the caller (see also signal(7) for more
details).
The behavior of the call is dependent on the value of how, as
follows.
SIG_BLOCK
The set of blocked signals is the union of the current set and
the set argument.
SIG_UNBLOCK
The signals in set are removed from the current set of blocked
signals. It is permissible to attempt to unblock a signal
which is not blocked.
SIG_SETMASK
The set of blocked signals is set to the argument set.
If oldset is non-NULL, the previous value of the signal mask is
stored in oldset.
If set is NULL, then the signal mask is unchanged (i.e., how is
ignored), but the current value of the signal mask is nevertheless
returned in oldset (if it is not NULL).
A set of functions for modifying and inspecting variables of type
sigset_t ("signal sets") is described in sigsetops(3).
The use of sigprocmask() is unspecified in a multithreaded process;
see pthread_sigmask(3).

EFAULT The set or oldset argument points outside the process's
allocated address space.
EINVAL Either the value specified in how was invalid or the kernel
does not support the size passed in sigsetsize.

It is not possible to block SIGKILL or SIGSTOP. Attempts to do so
are silently ignored.
Each of the threads in a process has its own signal mask.
A child created via fork(2) inherits a copy of its parent's signal
mask; the signal mask is preserved across execve(2).
If SIGBUS, SIGFPE, SIGILL, or SIGSEGV are generated while they are
blocked, the result is undefined, unless the signal was generated by
kill(2), sigqueue(3), or raise(3).
See sigsetops(3) for details on manipulating signal sets.
Note that it is permissible (although not very useful) to specify
both set and oldset as NULL.
C library/kernel differences
The kernel's definition of sigset_t differs in size from that used by
the C library. In this manual page, the former is referred to as
kernel_sigset_t (it is nevertheless named sigset_t in the kernel
sources).
The glibc wrapper function for sigprocmask() silently ignores
attempts to block the two real-time signals that are used internally
by the NPTL threading implementation. See nptl(7) for details.
The original Linux system call was named sigprocmask(). However,
with the addition of real-time signals in Linux 2.2, the fixed-size,
32-bit sigset_t (referred to as old_kernel_sigset_t in this manual
page) type supported by that system call was no longer fit for
purpose. Consequently, a new system call, rt_sigprocmask(), was
added to support an enlarged sigset_t type (referred to as
kernel_sigset_t in this manual page). The new system call takes a
fourth argument, size_t sigsetsize, which specifies the size in bytes
of the signal sets in set and oldset. This argument is currently
required to have a fixed architecture specific value (equal to
sizeof(kernel_sigset_t)).
The glibc sigprocmask() wrapper function hides these details from us,
transparently calling rt_sigprocmask() when the kernel provides it.

This page is part of release 4.13 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SIGPROCMASK(2)